1. Field of the Invention
The present invention relates to a stepper motor and, more particularly, to a rotor movement warning device for warning the user not to rotate the rotor when the stepper motor is in the stand-by position.
2. Description of the Prior Art
The stepper motor is a motor which rotates in short and essentially uniform angular movements rather than continuously. The angular movement is determined by the number of driving pulses applied to the stepper motor. Unlike the synchronous induction motor, the stepper motor can change the direction of the rotation by the change of exciting sequence of the windings in different phases. Also, the stepper motor differs from the two-phase servo motor such that the stepper motor does not require a closed loop control system, but requires an open loop control system, for the position detection. Also, in a stepper motor, the synchronizing force functions as a recovering force.
Therefore, in general, the stepper motor is driven by the open loop control system by counting the number of driving pulses applied to the stepper motor. When the stepper motor stops, taking a stand-by position ready for the further rotation, one or more windings in the particular phase or phases are applied with a constant current for generating a holding torque to maintain the rotor in the rotated position.
The step motor is used in various electric appliances, such as in a printer for moving the printer head. In such a case, an operator may try to forcibly move the printer head, resulting in undesirable displacement of the system. For example, during the exchange of the ink ribbon cassette or a printer wheel, an operator may forcibly shift the printer head which carries these ink ribbon cassette and the printer wheel to a position easy to do the exchange. When the force applied by the operator exceeds the holding torque of the stepper motor, the rotor of the stepper motor rotates without using any driving pulse. In such a case, the printer head will be shifted to a position which does not coincide with the location traced by counting the number of driving pulses.